Friend isn't able to join with nickname error

So I was able to get one friend into the server with cracked minecraft offline mode with same version. But when I tried to bring in another friend, it didn't work because his name was too big at first because it kept giving this error: "Internal Exception: io.netty.handler.codec.DecoderException: The received string length is longer than maximum allowed (19 > 16)". After looking it up, I assumed it was a nickname being too big problem which probably meant his nickname shouldn't be higher than 16 characters and that his name was Flashy_Imperfection at first. I told him to change probably several times already to no avail because it kept saying the same thing no matter what nickname he chose. I'm not sure what to do. Please help!

I was able to find out why it wasn't working for him. Turns out I was so blind I didn't see that he had to switch users after editing the profile name lol. Thanks for the help anyway.